How can ca-oxalate - rich food cause ca-oxalate stones in susceptible people when the ca is already bound to the oxalate before entering the body and doesn't readily reform?

Calcium oxalate in monohydrate form may be present in some foods. Whatever the formula of oxalate in food it will be broken down in our gut then oxalate by itself is absorbed, used by body and the extra is excreted by the kidneys. Inside the kidneys, oxalate can bind with calcium to form calcium oxalate when the media (pH, concentration ...) is ready for that then calcium oxalate stone develops.